This invention relates to hot melt adhesives which exhibit good high temperature performance and adhesion. In particular, this invention relates to hot melt butene-1 and ethylene copolymer adhesives. The present invention is an improvement over U.S. Pat. No. 4,568,713.
Hot melt adhesives can be formulated to be pressure sensitive and have an infinite open time but these adhesives are usually soft, tacky and have limited strength and adhesion. Conventional hot melts such as formulations of poly(ethylene/vinylacetate), polyethylenes, polyamides, or polyesters are rigid, form good strong bonds to certain substrates but have short open times usually less than 1 minute. Moreover, these adhesives usually have problems in adhering to cold metal substrates.
Solvent applied contact adhesives can be formulated to give good bond strengths and reasonable open times but since they require the use of solvents, which can cause toxicity, pollution, and fire hazards, solvent applied adhesives can have significant drawbacks. The instant polybutylene hot melt adhesives are unique in that they require no solvents, have good T-peel, good SAFT, and show improved adhesion to cold metal substrates, such as aluminum substrates.
The poly-1-butene polymers are a unique group of olefinic polymers because they crystallize very slowly. The very slow crystallization rate, in contrast to the crystallization rates of other polyolefin crystalline polymers, such as EVAs, polyethylenes and polypropylenes, has been found to provide long open times as well as good adhesion and bonding to cold, heat-sink type substrates--metals such as stainless steel and anodized aluminum, for example.
U.S. Pat. No. 4,568,713 teaches polybutene-based hot melt adhesives containing either poly-1-butene copolymers or poly-1-butene homopolymers and tackifying resins. This broad patent, does not identify certain unexpected results relative to SAFT and T-peel values obtained by incorporating certain carefully selected proportions of poly-1-butene homopolymer with poly-1-butene copolymer in adhesives blends containing a tackifying resin.
Polybutylene polymers usable in the present invention are composed of linear chain molecules with a regular and spacially ordered arrangement of ethyl side groups, the pendant groups that result when one butene is polymerized across the 1,2 carbon double bond (along an ethylene chain backbone) (see U.S. Pat. No. 3,362,940). When cooled from melt, the ethyl side groups initially align in a tetragonal spatial arrangement, developing a little over one half of the ultimate crystallinity (form II). With time, the tetragonal crystalline phase transforms into a stable hexagonal spatial arrangement with subsequent development of additional crystallinity (form I). This is a very slow process, the transformation being completed in the neat polymer over a period of several days.
Butene-1 can be copolymerized with a variety of alpha-olefins to provide useful copolymers such as those taught in U.S. Pat. No. 3,362,940. Butene-1/ethylene copolymers, with ethylene monomer in the 5.5-20 mole percent range have been taught in U.S. Pat. No. 4,568,713, and U.S. Pat. No. 3,362,940 and it has not been discovered that copolymers of this type with low ethylene comonomer contents, (1-6 w %) are of special interest in hot melt adhesives. The slight amount of ethylene comonomer may produce a lower glass transition temperature (T.sub.g) and increase the amount of amorphous phase in the polymer. The slight amount of ethylene may further reduce the crystallization rate. These polymers provide many advantages in the development of hot melt adhesives, such as a lower T.sub.g and a higher amorphous phase. These polymers offer wider formulating latitude in combination with compatible resins, waxes, oils, fillers and additives.